The present invention relates to a method of and to an apparatus for testing rod-shaped articles of the tobacco processing industry. More particularly, the invention relates to improvements in a method of and in an apparatus for testing the tubular wrappers of rod-shaped articles of the tobacco processing industry. Still more particularly, the invention relates to improvements in a method of and an apparatus for ascertaining the condition of wrappers by establishing a pressure differential between the interior and the exterior of the wrappers of successive articles, by detecting those changes of the pressure differential which are attributable to leaks in the wrappers, by generating first signals which are indicative of such changes of pressure differential, by monitoring a comparison or reference pressure, by generating second signals denoting the monitored comparison or reference pressure, and by thereupon processing the first and second signals to form test signals which are indicative of the quality of tested wrappers.
It is already known to practice a method of the above outlined character in an apparatus wherein a testing conveyor (particularly a rotary drum-shaped testing conveyor) transports a succession of rod-shaped articles of the tobacco processing industry sideways (i.e., at right angles to the axes of the respective articles) toward, through and beyond a testing station, wherein at least one end portion of the wrapper of the article advancing through the testing station is in engagement with a sealing element which is designed to prevent uncontrolled escape of testing fluid and/or uncontrolled admission of atmospheric air into the wrapper of the article at the testing station, wherein at least one conduit admits pressurized testing fluid into the wrapper of the article at the testing station so as to establish a pressure differential between the interior and the exterior of such wrapper, with signal generating pressure monitoring means for ascertaining the pressure of testing fluid which is being supplied into the wrapper of the article at the testing station, and with means for evaluating the signals which are transmitted by the monitoring means and for processing such signals to generate test signals denoting the condition of the wrappers of tested articles. Rod-shaped articles of the tobacco processing industry which can be tested in the just described apparatus include plain and filter cigarettes, filter rod sections, plain or filter cigarillos, plain or filter cigars, cheroots, papyrossi and/or other types of smokable articles or constituents of smokable articles wherein tobacco and/or filter material is confined in a tubular wrapper. For the sake of simplicity, the following description of conventional methods and apparatus will deal primarily or exclusively with the testing of filter cigarettes, especially filter cigarettes of the type wherein the wrapper portion surrounding the filter material is provided with one or more annuli of so-called ventilation holes serving to admit atmospheric air into the column of tobacco smoke.
As a rule, filter cigarettes as well as other types of rod-shaped articles of the tobacco processing industry are tested prior to discharging them from the maker (e.g., from a filter tipping machine) in order to ascertain the integrity or lack of integrity of the wrappers. The defects which are to be detected or which should be detected include, among others, insufficient or excessive permeability of the ventilating zone, open seams, frayed ends of the wrappers, holes or tears in the material of the wrappers and/or any possible combination of the just enumerated defects. As a rule, the testing operation involves the admission or evacuation of testing fluid at one end of the wrapper so as establish a pressure differential between the interior and the exterior of the wrapper, and monitoring of the thus established pressure differential. Deviations of the ascertained pressure differential from a reference value are indicative of defective wrappers, and the articles containing defective wrappers are normally segregated from other (satisfactory) articles if the deviation of the monitored pressure differential from a desired or optimum pressure differential exceeds a predetermined value. The pressure differential can be monitored and the signals which are generated by the monitoring means can be evaluated to ascertain the degree of ventilation as well as the presence or absence of other defects in the wrappers.
It is already known to eliminate or reduce the influence of drift phenomena in the pressure monitoring devices (e.g., electropneumatic transducers). Such drift phenomena can develop as a result of changes in temperature, contamination of the monitoring devices and/or of the pneumatic system of the testing apparatus and/or certain other undesirable influences. For example, it was proposed to carry out a reference pressure measurement following each testing operation and to subtract the thus obtained signals from the signals denoting the ascertained pressure differential during the preceding testing operation. As a rule, the reference pressure is atmospheric pressure. The signal which is indicative of the difference between the signal which denotes the pressure differential between the interior and the exterior of the wrapper of an article at the testing station and the signal which denotes the reference pressure is devoid of the influence of the aforediscussed drift phenomena, and such difference signal is then processed to obtain the desired test signal (e.g., a signal which can be used to effect segregation of the article having a defective wrapper from other (satisfactory) articles). Reference may be had to commonly owned U.S. Pat. No. 4,120,194.
A comparison of the pressure of testing fluid with the reference pressure can compensate for changes in the characteristics of the measuring instruments, such as pressure transducers. However, the compariosn fails to compensate for many other factors which adversely influence the results of measurements, i.e., which lead to misleading results and segregation of satisfactory articles or retention of defective articles among satisfactory articles. Thus, it is desirable that the drift compensation take place simultaneously with the step or steps of ensuring that the so-called admission pressure of testing fluid (namely the pressure of testing fluid which is being supplied into the wrapper of the article passing through the testing station) be maintained at a constant value. This involves considerable expenses and contributes significantly to the cost of the testing operation. On the other hand, fluctuations of such admission pressure (i.e., deviations from a constant pressure) can greatly influence the accuracy of the testing operation.